It is recognised in the art that non-variable valve duration is no more acceptable, from the point of view of engine efficiency, than would be non-variable ignition timing, or non-variable fuel metering.
Variable valve duration, especially when combined with variable valve lift, even when embodied in a form which tends to optimise duration and lift at only two infinitely narrow points on the speed range of an engine--as with Honda's current production two-stage system--demonstrates, among other advantages, that the superior torque curve that results allows addition of catalystic converters to control emissions without serious and wasteful loss of torque either side of the torque peak, as is the case with non-variable valve duration engines so equipped.
Obviously, valve duration and lift that is optimal at every point on the engine speed scale, and for all conditions of engine operation, would be proportionately superior to a two stage system as discussed, and by virtue of offering a predictable baseline of induction and exhaust control throughout the engine speed range, would offer opportunities to maximize fuel useage, and minimize polluting emissions, factors of crucial importance today and into the future.
Of the many systems proposed to achieve variable valve duration, one system that has often appeared is based on the principle of progressively disengaging the valve from the camlobe that drives it, thereby reducing the duration of the valve relative to the duration of the camlobe. Various mechanisms are proposed to achieve this; but suffer from the fact that, in reducing valve duration relative to camlobe duration, the opening and closing phases of camlobe profile are no longer in effect, with the result that the initial, and final, opening and closing phases, respectively, of valve motion are subject to unacceptable positive and negative acceleration rates, leading to rapid valve gear wear, and erratic valve action.
Accordingly, a system is proposed that compensates for the above mentioned problems.